(1) Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit. More particularly, it relates to an AGC circuit preferably applied to a variable gain amplifier operating as a receiver of radio wave signals having an abnormal signal-to-noise (S/N) ratio, such as radio wave signals transmitted via an artificial satellite transponder.
(2) Description of the Related Art
Recently, in radio communication systems, various information sets are transformed into digital signals and thus transformed information sets transmitted to a remote office through a digital radio transmission line. In such radio communication systems, the quality of the signal received is often deteriorated by the condition of the related radio transmission line.
To cope with this quality deterioration, a number of methods for modulating the transmission signals have been proposed; among which is the phase shift keying (PSK) method.
In a data transmitting-receiving system, received digital modulation signals modulated under, for example, the PSK method, are first applied to an AGC circuit for level stabilization and demodulated through a synchronous detection operation, and the original digital data is then reproduced. Obviously, in the above operation, the AGC circuit must operate normally, since it is located at the initial stage of the receiver unit. If the AGC operation is abnormal, the resultant reproduced digital data will necessarily contain errors.
However, a problem occurs in the prior art AGC circuit where the AGC circuit can no longer maintain a normal AGC operation when used for receiving signals having an abnormal S/N ratio. Such abnormal S/N ratio signals often occur in, for example, satellite communication systems.